That's most certainly caused by GarryG changing some of the NewGRFs recently: viewtopic.php?p=1259758#p1259758
When I saw this I was like since I had already spend days and weeks to place I don't know how many of these decorative elements and there is now way I will update those.
Either you delete the "rock objects" yourself or you "roll back" to the version of the AuzWaterObjects.grf that was used at the creation of the scenario, which is shown in the NewGRF options as "Auz Water Objects V15" and "Auz Water Objects Add Ons V9"
This "version hell" is exactly what I meant with "realiable" earlier.
There is an option to show all the available NewGRFs that are available, not just the latest version, but I forgot how to switch that, as it is not default - but it was the response of some experienced user to some of my other threads.